1. Field of the Invention:
This invention relates generally to millimeter wave devices and more particularly to millimeter wave circulators utilizing ferrite components.
2. Description of the Prior Art
A circulator is a high frequency device which directs electromagnetic input power therethrough in a non-reciprocal manner. Operation of a circulator may be compared to a turnstile having ports distributed about its circumference. Power entering the first port exits at the second port and power entering the second port exits at the third port, etc. so that the circulation of power through the circulator is consistently in the same direction. Circulators are used extensively in radar, for example, to provide isolation between outgoing power from transmitter (e.g. first port) to antenna (second port) and incoming power from the same antenna to the receiver (third port). The non-reciprocal property of a circulator is governed by a non-reciprocal element located at the junction of three (or more) transmission lines which form the ports of the circulator. Typically, the non-reciprocal element is a spinel type ferrite, such as LiZn or NiZn. In circulators employing spinel ferrites, it is necessary to bias the ferrite element with an external magnet, to achieve power flow in the preferred direction(s).
The biasing magnetic field is oriented in a direction perpendicular to the plane of the junction and the transmission lines. The direction of circulation can be reversed by reversing the direction of the biasing magnetic field. For example, in a three port circulator, if an upward orientation of the biasing magnetic field causes power to preferentially flow from port 1 to port 2 and from port 2 to port 3 and from port 3 to port 1, then a downward orientation of the biasing magnetic field would cause power to preferentially flow from port 1 to port 3, and from port 3 to port 2 and from port 2 to port 1.
With the advent of microelectronics, there is a growing need for high-frequency circulator devices that lend themselves to microwave and millimeter wave integrated circuit structures, such as microstrips. An example of a microstrip-compatible circulator is provided by U.S. Pat. No. 3,456,213, entitled "Single Ground Plane Junction Circulator Having Dielectric Substrate" issued to Hershenov. There is also a need for light, broadband circulators which require little or no external magnetic bias, thus reducing cost weight and complexity.